1. Field of the Invention
The present invention relates to a servo device for driving a controlled object based on input control signals. For example, the present invention relates to a servo device suitable for industrial radio control, radio control for models, remote control to robots, and the like, operated based on control signals transmitted with radio waves.
2. Description of the Prior Art
Radio controlled devices (hereinafter referred to as R/C) are in widespread use, as devices each which moves with control information carried with radio waves from a transmitter and at an area remote from an operator, or as devices each which manipulates a controlled body. The radio control devices generally manipulate controlled bodies including model cars, model air-planes, model ships, robots, and the like.
In such a radio control device, a servo system is equipped as an actuator that drives a control object in a controlled body. The servo system is mounted on the controlled body and drives the control object of the controlled body based on control signals, which are transmitted by a transmitter and received by a receiver.
FIG. 7 shows an outline of such a radio control device (system). A control panel 10 that manipulates an airborne object 13 or a controlled body includes joysticks and various setting switches.
An encoder 11, for example, pulse-width modulates various control signals output from the control panel 10, and converts them into a chain of pulses cycled in a predetermined frame period.
A pulse chain in one frame unit, cycled in a predetermined frame period, is always supplied to the high-frequency section 12 (transmission section) during manipulation. For example, AM or FM modulated radio waves are transmitted to an airborne object 13.
The radio control device includes the control panel 10, an encoder 11 and a high-frequency section 12.
FIG. 8 shows a pattern of a pulse chain cycled in one frame unit. The direction rotation (rudder or aileron) control signal, rise/fall (elevator) control signal, the speed (engine throttle) control signal and others in the airborne object 13 are converted into pulse signals CH1, CH2, CH3, CH4, . . . as channels 1, 2, 3, 4, . . . , respectively. One frame corresponds to, for example, a pulse chain repetitive in 14 mS to 20 mS.
In further explanation, the intervals Pw1 Pw2, Pw3, . . . of pulse signals CH1, CH2, CH3, . . . are changed based on plural sets of control information, respectively. For example, a change of ±600 μS (corresponding to a rotational angle of about 120° of the rotational shaft of a servo system, mounted on the airborne object 13, acting as an actuator that controls the movable members of the airborne object 13) occurs with respect to the center value of 1520 μS. A synchronous (space) signal of 5 mS is provided to indicate the end of one frame.
Control information of such a type is always transmitted with radio waves to the airborne object 13.
The airborne object 13, on the receiver side, receives the radio waves with, for example, a W super heterodyne receiver. A decoder performs the processing of received signals, demodulates control signals transmitted by the manipulator on the transmission side, separates control signals for respective channels, and supplies the separated signals to the servo system acting as an actuator.
FIG. 9(a) illustrates an outline of a decoder that separates and outputs control signals in respective channels from a chain of decoded pulse string signals (PPM). Numeral 21 represents a reset circuit that produces a detection output to a synchronous signal. Numerals 22, 23, 24, 25, . . . represent D flip-flops (hereinafter referred to as DFFs), respectively.
The demodulated pulse string signal PPM is input to the DFFs (22, 23, 24, 25, . . . ) forming a shift register and to the reset circuit 21.
When detecting an L-level duration of about 5 mS in the pulse string signal PPM, the reset circuit 21 produces a reset signal of a high level and then inputs it to the D input of the first shift register DFF 22. Subsequently, successive pulses are transferred to the shift registers. Thus, as shown with the waveforms in FIG. 7(b), the control signals CH1, CH2, CH3, CH4, . . . which respectively correspond to pulse intervals in a pulse string signal with modulated pulse positions, are output from each stage of the shift register to the servo device.
Here, explanation will be made as the fundamental configuration of the servo device.
The servo device is included in a housing. The housing includes a motor acting as a drive source for the servo device, a reduction gear for reducing and outputting the rotation of the motor, an output shaft for transmitting the output of the reduction gear to the control object of a controlled body, a variable resistor (potentiometer) for detecting a displacement of the output shaft, and a servo circuit for receiving control signals from a receiver and controlling the driving of the servo device.
The servo circuit creates motor drive signals based on input control signals and based on the displacement of the actuator due to the rotation of the output shaft detected by the variable resistor and thus outputs the motor drive signals to the motor to drive the motor controllably.
In the servo device, the motor acting as a drive source is a brushless dc motor (hereinafter referred to as a DC motor). The servo circuit is manufactured as a servo-system DC motor driving integrated circuit.
FIG. 10 is a block diagram illustrating the main portion of the servo-system DC motor driving integrated circuit. FIG. 11 shows signal waveforms A, B, C, D, E, and F in respective blocks.
The control pulse signal A in the channel i, extracted as shown in FIG. 11, is supplied to the comparator circuit 31 every one frame period. The control pulse signal A triggers the control position signal generator 39. The comparator circuit 31 compares the control pulse signal A with the sole position pulse signal B.
An exclusive OR circuit (comparator 31) is formed of inverters IN1 and IN2 and NOR circuits NR1 and NR2 and an OR circuit OR1. The exclusive OR circuit receives the control pulse signal A and the position pulse signal B output from the control position signal generator circuit 39. When the control pulse signal A has a pulse width wider than that of the position pulse signal B, the NOR circuit NR1 outputs at its output terminal the pulse C1 (hereinafter referred to as an error pulse signal) corresponding to the difference between the pulse widths of the pulse signals A and B, as shown with the letter C in FIGS. 10 and 11. Moreover, when the position pulse signal B has a pulse width wider than that of the control pulse signal A, the NOR circuit NR2 outputs at its output terminal the error pulse signal C2 corresponding to the difference between the pulse widths of the pulse signals A and B, as shown FIG. 11. The error pulse signal C1 is input to the set terminal S of the flip-flop 36 while the error pulse signal C2 is input to the reset terminal R of the flip-flop 36. As a result, the Q output terminal or the −Q output terminal becomes “1”.
Moreover, the dead pulse signal generator circuit (DGB circuit) 32 is triggered by the error pulse signals C1 and C2 via the OR circuit OR1 and thus produces a dead pulse signal D. When the error signal C is smaller than the dead pulse signal D, the voltage of the drive terminal of the DC motor M is held (at a difference voltage of zero) such that both the comparator 33 and the stretcher circuit 34 do not output any signal.
That operation prevents the servo device from being erroneously operated due to external forces or noises applied to the DC motor M for the servo device and provides a dead zone in the control system, thus effectively stabilizing the servo system.
The pulse width of the error signal E output from the comparator 33 is stretched in a predetermined ratio by the stretcher circuit 34. The stretcher circuit 34 outputs its stretched output pulse to the motor drive circuit 37 via the AND circuit AD1 or AD2 to rotate the DC motor M in a predetermined direction.
Thus, by defining the PWM signal width for control with the stretcher circuit 34, the gain characteristic of the servo circuit can be set.
The rotational direction switching circuit 35 changes the rotational direction of the motor according to whether or not the control pulse signal A is larger with respect to the current control position of the motor. For example, by comparing the current position pulse signal B from the control position signal generator circuit 39 and the control pulse signal A, the rotational direction switching circuit 35 can determine the (normal or reverse) rotational direction of the DC motor M.
In the servo device, the rotation of the DC motor M is transmitted to the output shaft via the reduction gear (not shown) to rotationally drive the output shaft.
The potentiometer PM cooperating with the operation of the output shaft outputs as an output signal the voltage indicating the rotational displacement (rotational position) of the output shaft from the potentiometer PM.
The control position signal generator circuit 39 pulse-width modulates the output signal of the potentiometer PM indicating the rotational displacement to produce the position pulse signal B. A change of the resistance value of the potentiometer PM is set to the direction where the position pulse signal B corresponds to the pulse width of the control pulse signal A. By doing so, the resistance value of the potentiometer PM is controlled over plural periods of the control signal received by the receiver. When the pulse width of an output pulse of the control pulse signal A matches with the output width of an output pulse of the position pulse signal B, the OR circuit OR1 does not output its output signal. Thus, the rotation of the DC motor M is stopped to drive the actuator to a target position. Finally, a closed loop of the servo circuit is formed based on the control pulse signal A.
The voltage (counter electromotive voltage) applied to the DC motor M is fed back to the control position signal generator circuit 39 via the resistor R1 to control the rotational speed of the DC motor M.
In the servo-system DC motor driving integrated circuit, the position pulse signal B output from the control position signal generator circuit 39 is narrowed with respect to the pulse width of the control pulse signal A indicating a target value as a pulse width, in the process of arriving at the target position. For the rotational speed adjustment, the voltage induced in the DC motor is fed back to an actual output pulse due to only the potentiometer PM via the resistor R.
The DC motor rotates such that the difference between the pulse width B of the position pulse signal and the control pulse signal A indicating the target position becomes zero. Thus, the DC motor M ceases at the target point.
As described above, the position servo control is performed such that a counter electromotive voltage component indicating the rotational information of the DC motor is fed back together with the signal indicating the rotational position of the DC motor. Thus, the motor can be controlled to decelerate and stop in front of a target position. This can prevent the motor from causing hunting through overrunning the stopping position due to the inertia of the motor.
As to the servo-system DC motor driving integrated circuit used in the conventional servo device, the above-mentioned control-only integrated circuit has been commercialized. Such a DC motor driving integrated circuit can be easily designed comparatively.
However, when the DC motor is continuously operated, for example, in a loaded state or the DC motor itself becomes a high temperature, there is the problem in that the operational life decreases noticeably due to abrasion of the commutating brush supplying current to the rotation coil.
Therefore, in the use of the DC motor to the servo device, there is the problem in that the DC motor always requires maintaining for high reliability.
In order to overcome such problems, it is considered to use, as a drive motor for the servo device, the brushless motor having a relatively high reliability and indicating the state more stabilized to noise signals. However, since such a motor is usually built so as to be rotationally driven with three or more phase drive pulses, even when the brushless motor only driving integrated circuit is used, it is particularly difficult to detect accurately the counter electromotive voltage. Hence, there is the problem in that that motor cannot be simply replaced with the drive motor in the servo device realizing the above-mentioned motor control.